Because in China workers are cheaper to hire, so they produce cheaper items and cheaper prices and make more of a profit.
The only way to verify if it's a fake is to go directly to the website of the original brand and either ask them or see what it is that distinguishes a fake from an original.
Most times the original will have either a stamp, signature or number that will identify it. Other times, the quality of the work or fabric, stitching, itself will show you. Sometimes it's obvious, while other times it's not.
It is the consumer's responsibility to determine the difference and the choice of whether to purchase or not. Where there is demand, there will always be product.